This invention is directed to a method for determining the injection source of a fluid flowing through a subterranean formation, particularly, the injection source of carbon dioxide in a carbon dioxide gas drive flooding project.
During the recovery of petroleum from subterranean formations, it is often necessary to inject a fluid into the subterranean formation to force the recoverable petroleum toward a producing well. Water flooding is a widely practiced example of a recovery process which is well known in the prior art. Other flooding processes such as steam flood, gas drive flood, carbon dioxide flood, and the like to enhance recovery of petroleum from subterranean formations are also known and practiced throughout the world.
Much effort and expense has been expended in developing and demonstrating the effectiveness of various subterranean formation flooding processes. Determining the flow patterns of the subterranean formation is very important to the success of a flooding project. As a consequence, tracers are injected into the formation for this purpose. The use of tracer materials for determining the fluid flow patterns of a subterranean formation is not new. Numerous materials are disclosed in U.S. Pat. No. 3,993,131 to Riedel which have been tried as tracers. The tracers include various dyes, gases such as helium and carbon dioxide, acids or the salts thereof, boron, bromide, chromate, dichromate, iodide, nitrate or thiocyanate ions, formaldehyde, carbon disulfide, and radioactive materials such as tritiated water (HTO), tritiated hydrogen (HT), tritiated methane, and krypton-85.
The use of these tracers has met with varying degrees of success and acceptability in the industry. The tremendous amount of activity and experimentation with different types of tracers is indicative of the great need that has existed for a relatively inexpensive and effective method of field evaluation of flooding processes in specific subterranean formations. Every formation has unique characteristics, therefore a need exists for an effective tracer process for determining flow patterns in the formation. One problem associated with the development of an effective tracer process is the appearance of an injected fluid at a producing well at a time other than that predicted. This is generally referred to as the breakthrough of injected fluid. Since a number of injection wells are usually spaced about the producing well, it is essential to determine the source of the injected fluid which will breakthrough at the production well. By identifying the source of the injected fluid, a determination of the flow characteristics of the subterranean formation between the injection and production wells can be made. For example, a quick breakthrough may be indicative of a higher than anticipated formation permeability or a fissure in the formation. Likewise, a breakthrough later than anticipated may be indicative of a lower than anticipated formation permeability or the presence of an obstruction in the formation. By identifying the source of the injected fluid, the flow characteristics of the formation about the production well can be mapped and adjustments made in the formation flooding process to maximize the recovery of petroleum from the formation.
An effective tracer material must flow with the flood front of the injected fluid and must be inert to the formation materials. By inert is meant that the tracer must not get adsorbed onto the rocks of the formation; it must not partition into the hydrocarbon or water phase; and it must not interact with the organics and minerals present in the formation. A tracer should also be safe to handle and reasonable in cost. The tracer must also behave in the formation as the fluid which is being traced and it must be detectable in the produced fluid, even in minute quantities.
Generally, the tracer systems available at the present time employ a slug injection technique. That is, a slug of the tracer material is injected into the formation followed by injection of the motive fluid. This is typical for injection of radioactive tracers. The high cost and safety hazards of some tracers prohibit periodic or continuous injection of the tracer in the motive fluid stream. For example, radioactive tracers are very expensive and require special handling by licensed personnel. The tracers used in the method of the present invention are relatively inexpensive, safe to use, and detectable and measurable at concentrations far lower than previously employed chemical tracers. The tracers utilized in the present invention may also be continuously injected in the motive fluid stream providing additional data indicative of the sweep efficiency of the flooding project.